1. Field of the Invention
The present invention relates to a system for controlling a driving force in a hybrid vehicle which is equipped as its motive force source with an internal combustion engine such as a gasoline engine or a Diesel engine and an electric motor such as a motor or a motor/generator to be activated by an electric power for outputting a torque.
2. Related Art
As well known in the art, the internal combustion engine inevitably emits exhaust gases. The components and amounts of the exhaust gases are dependent upon the running state of the internal combustion engine. At a high load running time when the throttle opening is increased at a low vehicle speed, according to a general tendency, it is liable to lower the cleanness of the exhaust gases and the fuel economy. In recent years, on the other hand, a demand for the cleanness of the exhaust gases of the vehicle having the internal combustion engine mounted thereon has grown higher and higher. In order to satisfy this demand, there has been developed a hybrid vehicle which is equipped with the internal combustion engine and an electric motor as its motive force source. In the hybrid vehicle of this kind, the motive force source can be selected according to the running state or the demanding state of the driving force so that the internal combustion engine can be run in the most efficient state. As a result, the fuel economy can be improved better than the prior art, and the emission of the so-called "greenhouse gases" can be reduced.
On the other hand, there is known as the type of the hybrid vehicle the so-called "parallel hybrid", in which an electric motor and a power generator are coupled to a power transmission line such as a transmission or a drive shaft and in which the internal combustion engine is selectively coupled to the power transmission line by a coupling mechanism such as a clutch so that the output of the internal combustion engine may be used as the motive powers for generating the electric power and for driving the hybrid vehicle. At a low vehicle speed time when the internal combustion engine drops its working efficiency or comes into an unstable running state, this parallel hybrid vehicle is driven to run by the electric motor, and the internal combustion engine is started when the vehicle speed rises to some level. In this case, the torque of the electric motor can be transmitted to the internal combustion engine through application means such as the clutch so that the internal combustion engine can be rotated and started if the application means is applied while the vehicle is driven to run by the electric motor. With this construction, the starter, as has been employed in the general vehicle of the prior art, can be done away with to reduce the size and weight of the drive unit.
When the internal combustion engine is not started before the vehicle speed rises to some extent, as described above, the coupling mechanism can be released at a low vehicle speed time or at a starting time to uncouple the internal combustion engine from the power transmission line so that the internal combustion engine can be left at a stop. A system for this control has been disclosed in Japanese Patent Laid-Open No. 37411/1997. According to this system, the output of the electric motor is not wasted even partially by the friction, as accompanying the mere rotation of the internal combustion engine, so that the power loss can be prevented to improve the energy efficiency.
Here, the internal combustion engine such as a gasoline engine or a Diesel engine can be fed with a fuel and ignited even in a state having a speed lower than the idle level. In this state of a low speed, however, the combustion is not stabilized to make the output torque or output rotation unstable. In the system, as disclosed in the aforementioned Laid-Open, the internal combustion engine is uncoupled at a low vehicle speed time from the power transmission line and is left at a stop. In addition to the aforementioned prevention of the power loss, therefore, it is possible to prevent the fluctuation in the driving torque and the resultant deterioration in the riding comfort.
However, the system of the prior art thus far described is constructed such that the internal combustion engine is completely stopped at the low vehicle speed time. When the vehicle is to be accelerated from the low speed running state, therefore, the vehicle is driven by the output of the electric motor till the speed of the electric motor rises to the level corresponding to the idle speed of the internal combustion engine, and the internal combustion engine is then coupled to the power transmission line and fed with the fuel so that it may be started. This makes it impossible for the internal combustion engine to output a high torque before the speed of the electric motor rises to the level corresponding to the idle speed of the internal combustion engine. As a result, it takes a long time to generate a driving force sufficient for a demand for an acceleration after the driver does an operation for the acceleration. This raises a disadvantage that the vehicle is deficient in a responsibility to the acceleration.
When the vehicle is to be started with a high running resistance as on a steep uphill, on the other hand, the vehicle speed cannot be raised if the output torque of the electric motor is relatively short. Since the internal combustion engine cannot be started in this case, the drive by the electric motor has to be continued but may be unable to satisfy the demand for the acceleration or may lower the charging amount of a battery abnormally.
According to the system of the prior art, moreover, when the vehicle is decelerated for a temporary stop, the internal combustion engine is uncoupled from the power transmission line to make the stop. When the vehicle is accelerated again after the temporary stop, however, the internal combustion engine cannot be started till the speed of the electric motor reaches the level corresponding to the idle speed of the internal combustion engine. This causes disadvantages that the vehicle is deficient in the acceleration and its responsibility.